1. Field of the Invention
The present invention relates to control valves for variable displacement pumps, and in particular for variable displacement radial piston pumps.
2. Description of the Related Art
Variable displacement radial piston pumps are useful in situations where the hydraulic demand on a system varies widely. The displacement of such pumps can be made to vary according to the pressure of hydraulic fluid in the pump crankcase, with the displacement increasing as the crankcase pressure decreases, and vice-versa. The pumps have the distinct advantage that they can be throttled back in low demand situations, thereby reducing their power consumption. Further details can be found in U.S. Pat. No. 3,002,462 (Raymond).
Unfortunately, these pumps have the disadvantage that they are inherently unstable, and this instability must be compensated for by the control circuitry for the pump. In particular, the pump increases stroke (flow) faster than it decreases stroke, because in order to decrease stroke the cam operating the pistons must make one complete revolution before the pump output flow is reduced. This built-in delay can cause significant feedback problems if appropriate control circuitry is not used.
One example of such control circuitry is found in U.S. Pat. No. 4,648,803 (Stephenson et al.). Stephenson et al.'s circuitry compensates for most of the inherent instability of the pump, but still leaves something to be desired. Stephenson et al.'s control valve adjusts its position based on the pressure differential between the pump crankcase and the pump outlet. When the outlet pressure peaks quickly, a sharp spike occurs in the crankcase and outlet pressures.
In Stephenson et al.'s circuit, this spike must be drained through the crankcase orifice. This means the crankcase orifice must be sized quite large to accommodate the spike, resulting in constant power drains due to the flow through the orifice in non-spike conditions.
In addition, Stephenson et al. provide a position IV in their main control valve to compensate for extreme spikes. However, the spikes required to move the control valve into this position IV are quite strong. The more normal spikes must be drained through the crankcase orifice.